Telecentric objective of the reversed telephoto type



pdated May 8, 1951.

Patented June 17, 1952 SEARCH ROO? TELECENTRIC OBJECTIVE F THE REVERSEDTELEPHOTO TYPE Max Reiss, Rochester, N. Y., assigner to Eastman KodakCompany, Rochester, N. Y., a corporation of New Jersey ApplicationFebruary 8, 1951, Serial No. 209,985

3 Claims.

rI'his invention relates to telecentric objectives such as used inoptical comparators and prole projectors.

Telecentric lenses were discovered by I. Porro in 1848 and independentlyby E. Abbe in 1878. The characteristic feature of these systems is thatthe position of the diaphragm is such that the principal rays on theshort conjugate side of the lens are parallel to the axis. The effect ofthis arrangement is that the object to be projected may be moved in andout of focus without changing its apparent size on the projectionscreen. Alternatively the object may have considerable thickness, andpart of it may be closer to the lens than the true conjugate plane andpart of it farther away. These parts will then be projected at the samemagnification regardless of the difference in their distance from thelens. Lenses of this kind are widely used by tool makers for testinggear wheels and similar objects at various stages of their manufacture.

'I'elecentric objectives of the reversed telephoto type arecharacterized by having a negative component facing the long conjugated.e., the projection screen) for increasing the clearance between theshort conjugate plane and the objective.

The present invention is a specic imprcve ment over the two telecentricobjectives shown as the last two examples in a copending application,Ser. No. 64,404, Turner and Kingslake, filed December 9, 1948, nowPatent 2,552,238, The objectives disclosed herein are speciiicallydesigned for use as interchangeable units in the system disclosed in theTurner and Kingslake application, and are also useful as interchangeabletelecentric objectives in ordinary profile projectors.

According to the present invention, a telecentric objective of thereversed telephoto type is made up of a field lens and an objectivesystem spaced apart so that the principal focal point of the field lensis located within the body of the objective system, wherein the eld lensconsists of a biconvex element cemented to the front of a negativemeniscus element and the objective system consists of a positivemeniscus element, a front biconcave element, a biconvex element, and arear biconcave element spaced apart and in that order, the positivemeniscus element being concave toward the rear, i. e., toward the firstbiconcave element.

I have discovered that by making the front element of the objectivesystem meniscus rather than by convex as in the Turner and Kingslakeexamples and by keeping the radii of curvature of the several surfaceswithin certain definite limits defined below, I have been able to makeup a series of telecentric objectives for use at a series ofmagnications from to 100 which are highly corrected for distortion,coma, curvature of field and spherical aberration and using onlyinexpensive types of glass readily obtainable on the market.

In some conditions of use, i. e., when means are provided for focusingby changing the distance between the object and the projection screen, asingle objective according to the invention is used throughout thisrange of magnication, and produces very clear and sharp images, but thedistortion will be found to change slightly at differentmagniiications-not enough to be noticeable, but enough to reduce theaccuracy of careful measurements.

For the purposes of the Turner and Kingslake invention, the distancebetween conjugate planes is xed, and an extremely high degree ofcorrection of the distortion is important so that measurements made onthe projected image are accurately applicable to the original object.Furthermore, a fixed series of magnications such as 10, 20, 31.25, 50,62.5 and is generally preferable to a continuously variablemagnification in machine shop practice since measurements of theprojected image are usually made by the ordinary scales divided intodecimal parts of an inch or into 16ths and 32nds and more often than notrelate to thousandths of an inch in the original object. In some shops amagnification of 39.37 for projecting a thousandth of an inch as amillimeter is very useful.

I have found that a very high degree of correction of the aberration andparticularly of distortion is obtained by making all six lens elementsof readily obtainable glasses having refractive indices between 1.59 and1.67, and by making the respective radii of curvature R1 to R11 of therespective surfaces numbering from front to rear, within the followinglimits:

where F' is the focal length of the objective as a whole and the andvalues of the radii indicate surfaces respectively convex and concave tothe front.

It may be noted that some of these radii appear to be specified within avery broadrange of values as compared with other radii. When it 1sremembered, however, that the effect a surface has on the focal lengthand on the aberrations of the whole objective is much more nearlyproportional to its curvature, i. e., the reciprocal of its radius ofcurvature, than to its radius of curvature, it will be seen that thelimits are roughly of' the same order of magnitude in all cases (interms of. the curvatures).

Within these broad ranges I find some particular relationships amongthek constants of the lens system to be particularly advantageous. Withrespect to the field lens, I find that the refractive index N2 of thenegative meniscus element should be between (N14-0.02) and (N14-0.04)Where N1 is the refractive index of the biconvex element cementedthereto. I find that an index dierence of about 0.03 at this cementedsurface iiattens the field very effectively and without introducing anundue amount of zonal distortion as is likely to occur if this indexdifference is chosen as greater than 0.04. Also, I find it convenient tochoose curvatures such that the sum of the powers of the two glassairsurfaces of the field lens is between 0.55 and 0.75.

I find also that the radii of curvature which give good results at onemagnification differ but little from those which give good results atanother magnification when expressed in terms of unit focal length.Slight adjustments are necessary, however, to accommodate for differentlens thicknesses and for slight changes in refractive indices. Theseslight adjustments are made in a manner well known among lens designers.

The thicknesses of the lens elements are chosen on the basis ofpractical considerations. There is a minimum thickness below which it isnot practical to work. In the case of negative elements this minimum isset by the tendency of the glass to bend during grinding and polishingand is about 0.4 mm. for lenses of mm. diameter and about 1 mm. forlenses of 10 mm. diameter. In the case of positive lenses, this minimumis set by the difficulty of polishing a lens element to a knife-edge andthe difiiculty of edge-grinding a thin-edged element without chipping.The edge thickness should be at least a millimeter for easy manufacture.Furthermore, any objective must be made up in a shorter focal length foruse at a higher magniiication when, as in the Turner and Kingslakesystem, the distance between the conjugate planes is fixed, andtelecentric objectives are no exception to this rule. It may be notedthat specifications are customarily given in terms of a focal length of100 mm. to facilitate comparitory thickness can be chosen for each ofthe three negative elements which remains at substantially the samefraction of the focal length in all the different examples designed fordifferent focal lengths. This is practicable because the highermagnification objectives cover smaller fields, hence the diameters ofthe elements are smaller, hence the minimum practicalthickness (inmillimeters) is smaller. The advantage of maintaining these thicknessesat substantially the same fraction of the focal length lies in the factthat less final readiustment of radii of curvature is required whenadapting a design for use at a. different magnification. Notwithstandingthe above, I find that a judicious change in the thickness of thenegative meniscus element is advantageous in balancing the effect on theaberrations of a change in the thickness of the biconvex elementcemented thereto.

In regard to the thicknesses of the three positive elements, I havefound that suitably chosen thicknesses for objectives designed for usein the lower and middle parts of the above-described range ofmagniiications lead to undesirably thin elements when scaled. down to ashorter focal length for the highest magnification, and so itl Aisadvantageous to make the positive elements relatively thicker in thelatter case. To partially compensate (aberration-wise) for the increasedthickness of the cemented positive element. I find that a slightdecrease in the thickness of the cemented negative element and a slightdecrease in the first airspace are advantageous. Also I find that anincrease in the l thickness of the meniscus positive element is roughlycompensated by a decrease of about sixtenths as much in the secondairspace, and that an increase in thickness of the rear positive elementis roughly compensated by a decrease of about one-third as much in thethird airspace plus a decrease of about one-sixth as much-in the fourthairspace.

In View of the practical considerations previously described and in viewof these interrelationships among the thicknesses and spaces, I havefound it desirable to maintain each thickness t and each space s (eachnumbered by subscripts from front to rear) within the limits specifiedas follows:

however, and I have found that the lateral color correction can bemaintained by choosing a glass of different dispersive index for therear negative element. Although only a rough rule can be given for thisdispersive index, I find that the optimum value varies more nearly asthe square root of the magnification than as the magnification itself,and that the dispersive index Ve i this element should be between(-2.5\/M) and (72-2.5\/M) for best results. Thus. when the magnificationat which the objective is intended to work is 10, Ve should be roughlybetween 57 and 64; when the magnification is 50, Vs should be between 47and 54; and when the magnification is 100, Ve should be between 40 and47. Although it is not feasible to have a special glass made up in eachcase to obtain a dispersive index exactly in the gmbh ou middle of thisspecified range. a series of glasses is commercially available havingrefractive indices between 1.60 and 1.62 and dispersive indicesincreasing from glass to glass in steps of less than 6 points throughoutthe range from 36.6 to 60.3. There is at least one glass of this serieswhich falls between the specied limits for any magnification from to100.

'I'he change to a different glass to obtain a favorable dispersive indexinvolves a moderate change in refractive index in most cases. As thefinal step in the design of an objective according to the invention, theradii of curvature are adjusted slightly. as is commonly done in lensdesigning, to reduce the residual aberrations to within acceptabletolerances.

In the accompanying drawing:

Fig. 1 shows in diagrammatic axial section an objective according to theinvention.

Figs. 2 and 3 give data for two embodiments thereof.

As shown in Fig. 1 the iield lens is made up of two elements I, 2cemented together, and the objective system is made up of a positivemeniscus element 3, a front biconcave element I, a biconvex element 5,and a rear biconcave element i. The radii of curvature R of therespective lens surfaces, the thicknesses t of the lens elements. andthe airspaces s between components are` each numbered by subscripts fromfront to rear.

Figs. 2 and 3 give data for telecentric objectives designed for use at10X and at 100x magnification respectively. These tables are repeatedbelow as the first and last of seven examples of telecentric objectivesaccording to the invention. In Figs. 2 and 3 and in the tables below,the lens elements are designated by number in the first column, thecorresponding refractive index N for the D line of the spectrum and thedispersive index V are given in the second and third columns, and theradii, thicknesses, and spaces are given in the last two columns. Theand values of radii R denote surfaces respectively convex and concavetoward the front. The linear dimensions Yhave been scaled up in eachcase to correspond to the customary focal length of 100 mm. tofacilitate comparisons.

[Example l, Fig. 2. Magniiication==101 Lens N V Radll Thicknesses Mm.Mm.

l 1.620 60.3 R1=+243.1 t1=28.0 2 1.649 33.8 Rz 45.78 tz= 7.5 R4 =178. 7a1=26.3 3 1.620 60.3 R4 45.04 ia=l68 R5 =+909.7 81=25.6 4 1. 617 36.6 R541. t4= 6.3 R7 52.92 sj=22 4 5 1. 620 60.3 Rg ==+203.2 t5==14.7 Ru 50.71 a4=39. 3 6 1.620 60. 3 Rm=475-0 In: 6.3

[Example 2. Magnioatlon==l Lens N V Radii Thlcknesses Mm. Mm.

1 1.620 60.3 Rx =+170.1 t|=28.4 2 1. 649 33.8 Rz 45.70 l4= 7.6 Ra =-216.5 s|=24. 5 3 1.620 60.3 R4 45.93 t3=17.0 R5 =+1247. sz=26. l 4 1. 61736.6 Re 39.70 4= 6.4 R1 52. 57 81=22. 7 1. 620 60. 3 Rx =+206. 6 t5=14.7 Rn 51. 28 s4=39. 7 1.617 55.0 R|o=332.9 t4: 6.4

[Example 3 Magnification 31.25]

Thick- Lens N V Radii messes Mm. Mm. 1 1.620 60.3 R1 -+159. 6 4:28. 22 1. 640 33.8 R4 --4626 t4- 7.5 R: --2631 s\==24.9 3 1. 620 60.3 R4 44.78 4=17.0 R4 =+719.0 .gp-25. 4 4 1.617 36.6 114-- 40.26 t4- 6.4 R1 51.09.1F-225 5 1.620 60.3 R4 =+204. 6 t5=l4.8 R 51.09 .s4-39.6 6 1. 617 55.0Riv- 543.3 t4- 6.4

[Example 4 Magniilcatlon=39.37]

Thick- Lens N V Radil messes Mm. Mm. 1.620 60.3 R1 =+159.6 11262 i. 64933.8 Rz 46.16 t4= 7.5 Rs =232. 6 s1==24. 9 1. 620 60.3 R4 I+ 44.811=16.9 R5 ==+717. 2 .n==25. 4 1. 617 36. 6 Re 40. 37 l4= 6. 4 R4 51.08sz=22.5 5 1.620 60.3 R4 :+2046 t5=14. 8 Ro 51. 08 84==39. 6 1.617 55.0Rmx-543.7 t4- 6.4

[Example 5 Magnification 50] Lens N V Radii Thleknesses Mm. Mm. 1. 62060. 3 R4 =+159.9 t1=28. 3 l. 649 33.8 R4 46.36 t4= 7.6 R: =IZ32. S813250 1. 620 60. 3 R4 45. 47 ;=16. 9 R5 =+955. 4 a4=25. 5 1.617 36.6 R4=40.03 14- 6.4 R1 I+ 51.94 s3=22. 5 1. 620 60.3 Rx :+187 8 t5=14.8 Re52. 21 84:39. 6 1. 616 49. 2 Rmx-589. 9 tu: 6. 4 R11=+355. 2

[Example 6 Magnication=62.5]

Lens N V Radll Thicknesses Mm. Mm. 1.620 60.3 Ri 157.9 t4=27.9 1.64933.8 Rz 46.0 tpl 7.4 R; 231.6 a4=26.3 3 1. 620 60.3 R4 46.01 t4=19.3 R5=+l900. 3F24. 0 4 1. 617 36.6 R4 39. 26 t4: 6.3 R1 52. 25 .41:22. 4 5 1.620 60.3 R4 199.0 t;=14.6 o 50.90 a4=39.3 6 1.616 49.2 Rm:- 53114 t4:6.3

[Example 7, Fig. 3 Magnication=l00.]

Lens N V Radil Thicknesses Mm. Mm. 1 1. 620 60.3 R1=+1B8.3 t|=40.3 2 1.649 33.8 RF1-43. 56 l4=6. 4

R4==192. 7 31=24.3 3 1.620 60.3 R|=+44.20 t4=29.4 R5=+928 5 3z=18. 24 1. 617 36. 6 Enr-38. 91 t4=6.1

` 11124-50. 62 s;=20. 4 5 l. 620 60. 3 R41-+178. 4 6F20. 8

Re: -5l. 02 .94138. 5 l 6 l. 605 43. 6 RWS-614. 8 4261.

tion. By conjugate planes which are optically 7 '768 mm. apart" is meant.that the distance from the short conjugate plane to the apparentposition of the long-conjugate plane as viewed from the rearsurface ofthe telecentric objective is 768 mm. The actual distance is slightlydierent, due to the effect of the Fresnel-type lens. The distortion ofthese systems is corrected to a residual value whichcounterbalances thedistortion due to the Fresnel-type lens.

Although, as previously mentioned, the focal lengths are given here as100 mm. in each case, the focal lengths when made up for use asinterchangeable lenses in the manner described are as follows:

The linear dimensions given above are to be multiplied by a constant ineach case in a manner too well known to need description, and byproportionately smaller or larger constants when made up for use betweenconjugate planes that are closer together or farther apart than thedistance above mentioned.

It will be directly evident from inspection of these tables that eachradius of curvature R, each thickness t, and each space s in all theexamples is within the specified range according to the invention. Thevalue of (2.5\/M|V7) is approximately as follows in the seven examples:

Example These values are between 65 and 72 in all examples in accordancewith this feature of the invention and each objective is well correctedfor color when used with a Fresnel-type field lens at the long conjugateplane as described in the previously mentioned copending application.

I claim:

1. A telecentric objective of the reversed telephoto type adapted to beused at a magnification between 10 and 100 inclusive and corrected forspherical aberration, coma, distortion, curvature of field, and lateralcolor at said magnification, comprising a field lens in front, that isfacing the shorter conjugate, and an objective system axially alignedand spaced therebehind, in which the field lens consists of a biconvexelement cemented to the front of a meniscus negative element whoserefractive index is greater than that of the biconvex element by between0.02 and 0.04, and in which the objective system consists ofAa-'positive meniscus element convex to the front, a front biconcaveelement, `a biconvex element and a rear biconcave elementairspaced'apart in that'order from front to rear, in which the radii ofcurvature R of the lens surfaces, the thicknesses t of the `8 lenselements, and the airspaces s, each numbered by subscripts from front torear, arenumerically within the limits set forth in the following tableof inequalities:

where F is the focal length of the telecentric objective, and in whichthe refractive index of each element is between 1.59 and 1.67.

2. A telecentric objective according to claim 1 constructedsubstantially according to the following specifications:

Lens N V Radii Thicknesses 1 1.62 60.3 R1 =+2.4F t|=.28F

2 l. 65 33. 8 R2 46F- h=. 08F

3 1. 62 60. 3 R4 45F t3=.17F

4 1.62 36.6 Rt .42F t4=.06F

5 1. 62 60. 3 Ra =+2. 0F t5=.15F

6 1. 62 60. 3 R1o= 4. 8F t=. 06F

Rn=4. 2F

where the lens elements are numbered from front to rear, N is therefractive index for the D line of the spectrum, V is the conventionaldispersive index, and the and values of radii R denote surfacesrespectively convex and concave to the front.

3. A telecentric objective according to claim 1 constructedsubstantially according to the following specications:

where the lens elements are numbered from front to rear, N is therefractive index for the D line of the spectrum, V is the conventionaldispersive index, and the and values of radii R denote surfacesrespectively convex and concave to the front.

MAX REISS.

(References on following' page) 2,aoo,aou

REFERENCES CITED Number The following references are of record in the2380310 le of this patent: UNITED STATES PATENTS 5 Number Name Date2,256,228 Zimmermann Sept. 16, 1941 Number 2,313,460 Warmisham Mar. 9,1943 444,350

SEARCH KUl Name Date Bennett July 10, 1945 Warmisham et al. Dec. 18,1945 Wynne Feb. 12, 1946 FOREIGN PATENTS Country Date Great Britain Mar.19, 1936

